This invention relates generally to storage systems associated with computer systems and more particularly to providing a method and apparatus for providing logical volumes which include more than one physical storage device.
As it is known in the art, computer systems generally include a central processing unit, a memory subsystem and a storage subsystem. According to a networked or enterprise model of a computer system, the storage subsystem associated with or in addition to a local computer system, may include a large number of independent storage devices or disks housed in a single enclosure. This array of storage devices is typically connected to several computers (or hosts) via dedicated cabling or via a network. Such a model allows for the centralization of data which is to be shared among many users and also allows a single point of maintenance for the storage functions associated with the many computer systems.
One type of storage system known in the art is one which includes a number of disk storage devices configured as an array (sometimes referred to as RAID). Such a system may include several arrays of storage devices. In addition to the arrays of storage devices, typical storage systems include several types of controllers, such as host controllers and disk controllers, for controlling the various aspects of the data transfers associated with the storage system.
The disk devices of a storage system are typically configured to represent one or more so called logical device. A logical device is a way to define a contiguous area of storage space as being available as a distinct addressable unit. The addresses used to access data in a logical device typically need to be translated into physical addresses in order to find the requested data. In many systems, a logical device includes all the addressable storage of a single physical volume (e.g. disk drive). Thus, state of the art storage systems may include logical devices that provide 9 GB of storage (from a single disk drive).
Several operating systems (e.g. Microsoft.RTM. Windows N/T) require that the data storage systems support logical device sizes which exceed current physical volume sizes. One approach to providing larger logical devices would be to increase the size of the associated physical volumes. This would be accomplished by, for example, replacing a 9 GB disk drive with a 23 GB disk drive. This approach suffers from several drawbacks including the increased expense associated with larger capacity drives and the need to retrofit installed systems with these drives in order to provide the larger logical devices. Additionally, in storage systems like the SYMMETRIX.RTM. storage system manufactured by EMC Corporation of Hopkinton, Mass., using a larger physical volumes requires increasing the size of each associated device header in global memory. This requirement arises from the fact that there would then be more tracks associated with each logical device. It would be advantageous therefore to provide a means of using two or more physical volumes to define a single large logical device. It would be of further advantage that the arrangement of the new large logical device be transparent to a disk controller controlling the physical volumes associated with the large logical device.